The invention relates to a method of manufacturing a patterned layer of a cholesterically ordered polymer material, in which the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, and the layer is patterned in that it has at least an area in which the pitch of the molecular helix differs from that of another area.
The invention also relates to a layer of a cholesterically ordered polymer material.
The invention further relates to an information carrier provided with a patterned layer of a polymer material having a cholesteric order.
The invention further relates to a polarization conversion system provided with a patterned layer of a cholesterically ordered material.
The invention further relates to a cholesteric color filter having a patterned layer of a cholesterically ordered material.
The method of the type described in the opening paragraph is known per se. For example, United Kingdom patent specification GB 2,314,167 describes a patterned layer of a cholesterically ordered material. In accordance with this patent specification, such a layer may be manufactured by first providing a uniform layer of a cholesteric material on a substrate. By polymerizing areas of this layer at different temperatures, a patterned cholesteric layer is obtained. Use is made of the fact that the pitch of the molecular helix of the cholesterically ordered material is temperature-dependent. By polymerizing areas of the layer at a given temperature, the pitch associated with this temperature is, as it were, frozen in these areas.
The known method has drawbacks. For example, in practice it has been found that the known method is difficult to implement. This notably applies to the case where more than two areas having mutually different pitches must be provided in the layer. In that case, a relatively large number of masking steps is necessary and the precision with which the masks are adjusted is very critical. Moreover, the maximum difference in pitch which can be realized between the different areas by means of the known method appears to be relatively small. Patterning at different temperatures also appears to be difficult in practice.
It is an object of the invention to obviate the known drawbacks. More particularly, it is an object of the invention to provide a method in which the layer can be patterned at the same temperature and in which relatively large pitch differences between the different areas can be realized. It is a further object of the invention to provide a polarization conversion system having a patterned layer of a cholesterically ordered material, and a cholesteric color filter having a patterned layer of a cholesterically ordered material, manufactured by means of this method.
These and other objects of the invention are achieved by means of a method of the type described in the opening paragraph, wherein the method comprises the steps of:
a. providing a layer of a cholesterically ordered material comprising a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, in which the conversion of said compound may be induced by radiation,
b. irradiating the layer in accordance with a desired pattern so that at least a part of the convertible compound in the irradiated parts of the layer is converted,
c. polymerizing and/or crosslinking the cholesterically ordered material to form a three-dimensional polymer.
It has been found that, using the method according to the invention, that patterned layers of cholesterically ordered, liquid crystalline material can be manufactured in a simple way at the same temperature, with the maximum pitch difference between the areas being relatively large. By (partially) converting the convertible compound in the irradiated areas of the layer, the pitch of the molecular helix in the layer is altered in these areas. The conversion of the convertible compound is effected by irradiation with energy in the form of, for example, electromagnetic radiation, nuclear radiation or an electron beam. Preferably said conversion is effected by means of UV radiation. The pitch of the molecular helix in the irradiated parts differs from the pitch of the molecular helix in the non-irradiated parts of the layer. By polymerizing and/or crosslinking the patterned layer thus obtained, the pitch in the different layer parts is frozen, as it were, and said pitch remains fixed during further process steps, storage and use of the patterned layer. In this way, a patterned layer of cholesterically ordered material can be manufactured in a simple manner.
The degree of conversion of the convertible compound in a certain area is determined by the irradiation dose in said area. Consequently, the pitch of the molecular helix is determined by the local irradiation dose. Said pitch of the molecular helix determines the local optical properties.
It is to be noted that, preferably, the cholesteric layer has a low absorbance for the radiation used in step b, and the radiation intensity along the axis of the helix (i.e. transverse to the layer) is relatively constant in each area. Consequently, the irradiation dose transverse to the layer is relatively constant, and therefore the value of the pitch, viewed along the axis of the helix, is relatively constant in each area. However, this value may differ for the different areas obtained by patterning. Viewed in the plane of the layer, the different areas are adjacent to each other, not subjacent.
When the cholesteric layer has a high absorbance for the radiation used in step b, the radiation intensity will show a gradient transverse to the layer according to Beer-Lambert""s law. Consequently, the top of the layer will receive more radiation than the bottom of the layer. This will lead to the formation of a gradient in the pitch, viewed along the axis of the helix (i.e. transverse to the layer). The presence of an absorbing material in a non-absorbing cholesteric layer yields also a gradient in the pitch.
A variation in the pitch, transverse to the cholesteric layer, can be obtained by a method of manufacturing a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, wherein the method comprises the following steps:
a. providing a layer comprising a cholesterically ordered material, which material comprises a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, the conversion of said compound being inducible by radiation, and the layer substantially absorbs said radiation.
b. irradiating the layer so that at least a part of the convertible compound in the irradiated parts of the layer is converted,
c. polymerizing and/or crosslinking the cholesterically ordered material to form a three-dimensional polymer.
According to the prior art, a gradient in the pitch transverse to the layer could be obtained by a method described in the U.S. Pat. No. 5,793,456 (PHN 14.629), which discloses a method of manufacturing a cholesteric polarizer by providing a mixture of chiral and nematogenic monomers, each having a different reactivity, in the form of a layer. The pitch of the molecular helix is governed to an important degree by the ratio between the chiral and the mesogenic monomer in the polymer material. Owing to the difference in reactivity between both monomers, the capture probability of the most reactive monomer is greater than that of the least reactive monomer. If during the polymerization of the mixture, which is initiated by actinic radiation, a variation in the radiation intensity is realized across the optically active layer to be formed, the most reactive monomer is preferably incorporated in the polymer at the locations of the highest radiation intensity. As a result, one or more concentration gradients of free monomer are formed during said polymerization process, causing monomer diffusion from locations with a low monomer concentration to locations with a high monomer concentration. This leads to an increase in reactive monomers in areas of the formed polymer material where, during the polymerization process, the radiation intensity was highest. As a result, the composition of the polymer material varies in the direction transverse to the polymer layer, causing a variation in the pitch of the molecular helix in the layer, which molecular helix is formed by the polymer. This variation of the pitch provides the optically active layer with a large bandwidth, the value of which is proportional to the value of the variation in pitch.
The known method of U.S. Pat. No. 5,793,456 has drawbacks. For example, the speed of the process is governed by the diffusion of the monomer molecules which is an inherently slow process. The present invention provides a rapid method in which a pitch of the molecular helix can be made to vary transversely to the layer. An additional advantage of the method according to the invention is that the pitch of the cholesterically ordered material is determined by the irradiation dose instead of the irradiation intensity. A certain irradiation dose can be administered in a short timeframe using a high irradiation intensity.
A combination of the method according to claim 15 and the method described in U.S. Pat. No. 5,793,456 is also possible. In said combination the effects of photo-isomerisation and diffusion are combined in order to obtain an even more increased gradient in the pitch of the molecular helix. If during the polymerization of a mixture of a photo-isomerisable chiral compound and a reactive nematogenic monomer, which polymerization is initiated by actinic radiation, a variation in the radiation intensity is realized across the optically active layer to be formed, two effects will cause a gradient in the pitch of the molecular helix. Firstly, due to the variation in the radiation intensity the amount of photo-isomerisable material that is converted will show the same variation across the thickness of the layer. Because the converted state determines the pitch of the cholesterically ordered material to a different extent, said variation in converted material will yield a gradient in the pitch of the molecular helix. Secondly, the reactive nematogenic monomer is preferably incorporated in the polymer at the locations with the highest radiation intensity, as previously described in the third paragraph. As a result, the composition of the polymer material varies in the direction transverse to the polymer layer, causing a gradient in the pitch of the molecular helix, and at the end of the process, the cholesteric layer is crosslinked. When using the proper materials, the gradients of both effects add up to a substantial gradient in the pitch of the molecular helix, in a crosslinked layer, in a short time frame.
Preferably, the photo-isomerisable chiral compound is less reactive than the nematogenic monomer. In this sense, the photo-isomerisable chiral compound may be a chiral acrylate, or even a non-reactive compound, and the reactive nematogenic monomer may be a nematic diacrylate.
It has been found that, using the method according to claim 15, broadband polarizers of cholesterically ordered, liquid crystalline material can be manufactured in a simple way. By (partially) converting the convertible compound in the irradiated areas of the layer, the pitch of the molecular helix in the layer is altered in these areas. The conversion of the convertible compound is preferably effected by means of UV radiation. The degree of conversion of the convertible compound is determined by the irradiation dose. Consequently, the pitch of the molecular helix is determined by the local irradiation dose. Said pitch of the molecular helix determines the local optical properties. By polymerizing and/or crosslinking the layer thus obtained, the pitch in the different layer parts is frozen, as it were. In this way, a layer of cholesterically ordered material can be manufactured in a simple manner.
In contrast, when the cholesteric layer has a low absorbance for the radiation used in step b, the radiation intensity along the axis of the helix (i.e. transverse to the layer), is relatively constant over the cross-section of the layer. Consequently, the irradiation dose transverse to the layer is relatively constant, and therefore the value of the pitch, viewed along the axis of the helix, is relatively constant in each area, yielding a small bandwidth (approximately 60-70 nm).
An embodiment of the method according to claim 3 wherein the irradiation dose in accordance with step b decreases from the top to the bottom of the cholesteric layer, is characterized in that, at the bottom of the cholesteric layer, said irradiation dose is less than 0.9 times the irradiation dose at the top of said layer.
When the cholesteric layer is caused to absorb the radiation used in accordance with step b, the radiation intensity will show a gradient transverse to the layer according to Beer-Lambert""s law. Consequently, the top of the layer will receive more radiation than the bottom of the layer. Said variance in the irradiation dose over the cross-section of the layer will lead to the formation of a gradient in the pitch of the molecular helix, viewed along the axis of the helix (i.e. transverse to the layer). This gradient in the pitch provides the optically active layer with a larger bandwidth, the value of which is proportional to the value of the variation in the pitch. When the cholesteric material""s absorbance of the radiation used in method step b is to small to produce the desired gradient in the pitch of the molecular helix in a certain amount of time, an absorbing material may be added to the cholesteric layer to obtain the required absorbance.
A preferred embodiment of the method according to claim 3 is characterized in that, at the bottom of the cholesteric layer, the irradiation dose in accordance with step b is less than 0.75 times the irradiation dose at the top of said layer. Said preferred variation of the irradiation dose across the thickness of the cholesteric layer yields a reflection band, which may span a substantial part of the visible spectrum.
From the description above, it is obvious that the pitch of the molecular helix in a layer of cholesterically ordered material can be suitably adjusted, yielding materials with special optical properties.
An interesting embodiment of the method according to the invention is characterized in that irradiation in accordance with step b is performed such that the irradiation dose is different for at least two different areas of the layer. By using this measure, it is possible to manufacture patterned cholesterically ordered layers which have juxtaposed areas, as viewed in the plane of the layer, with a different value of the pitch of the molecular helix. The different irradiation doses may be realized by using different irradiation periods at a substantially constant intensity. Alternatively, the different irradiation doses may be realized by using a higher irradiation intensity at a substantially constant irradiation period.
Preferably, however, masks are used having a number of apertures which have a different transmissivity to the radiation used. In that case, three or more areas in which the pitch of the molecular helix is different can be obtained in one irradiation step, using one mask.
In principle, a large number of types of convertible compounds influencing the pitch of the molecular helix of cholesterically ordered material may be used within the scope of the invention. In the first place, convertible chiral compounds are feasible, which, due to irradiation, fall apart into non-chiral compounds. The presence of chiral compounds promotes the formation of a cholesteric ordering in a liquid crystalline solution. Irradiation of selected areas of a cholesterically ordered layer with decomposable chiral compounds leads to an increase of the pitch of the molecular helix in these areas.
Another advantageous embodiment of the method according to the invention is characterized in that the convertible compound comprises an isomerizable, chiral compound. The different isomeric forms of such an isomerizable chiral compound usually have a different influence on the pitch of the molecular helix of the cholesterically ordered material. By locally changing the ratio of these isomeric forms by way of irradiation, the pitch is changed. This provides an elegant possibility of manufacturing patterned layers of a polymer material with a cholesteric ordering and a different pitch. To prevent diffusion of the isomerizable, chiral compound in the patterned layer, this compound is preferably bound via a chemical bond to the liquid crystalline polymer material having the cholesteric order. In the latter case, the UV stability of the patterned layer also appears to have been improved.
The cholesterically ordered material used in the method according to the invention comprises liquid crystalline monomers, liquid crystalline oligomers and/or liquid crystalline linear polymers with reactive groups. Due to the presence of these reactive groups, this material can be converted into a polymer material by polymerization and/or into a three-dimensional molecular network by crosslinking. For the reactive groups, notably epoxy groups, vinyl ether groups and/or thiolene groups are suitable. Particularly suitable reactive groups are those of the (meth)acrylate type. It has been found that cholesterically ordered polymer layers having a high optical quality can be obtained with these types of reactive groups. It is to be noted that, when using linear polymers, only crosslinking is necessary for obtaining a three-dimensional network. However, when monomers and/or oligomers are used, polymerization and crosslinking should take place for obtaining the envisaged three-dimensional molecular network. The stabilizing of the cholesteric layer in process step c after selective adjustment of the pitch of the cholesterically ordered layer in process step b, is an important step in the method according to the invention.
An embodiment of the method according to the invention is therefore characterized in that the polymerization and/or crosslinking is initialized and/or catalyzed by the addition of an initiator or catalyst from the fluid or gaseous phase. Said addition is preferably performed after steps a en b in accordance with the invention in order to prevent a polymerization and/or crosslinking reaction during steps a and b. Various initiators and catalysts are applicable.
A further embodiment of the method according to the invention is therefore characterized in that polymerization and/or crosslinking is induced by a thermally decomposable initiator. In that case, the layer of cholesterically ordered material preferably comprises a small quantity of a thermally decomposable polymerization initiator. Said initiator is inactive during process step b according to the invention. Subsequently, the polymerization and/or crosslinking of process step c, may be effected by activating the initiator at an elevated temperature.
An embodiment of the method according to the invention is characterized in that polymerization and/or crosslinking is effected by means of electron-beam irradiation. Very hard layers can be manufactured by means of this method. In this variant of the method according to the invention, it is not necessary to use a polymerization initiator.
An embodiment of the method according to the invention is characterized in that polymerization and/or crosslinking is effected by exposure to actinic radiation. The polymerization and/or crosslinking of a layer of the cholesterically ordered material (step c) can take place in the presence of a photo-initiator by using actinic radiation such as UV radiation. An advantage of using photo-polymerization is that this method permits local polymerization and/or crosslinking in very small areas.
Since the conversion of the convertible compound (step b) is also preferably effected by means of UV radiation, step b and step c of the method claimed may interfere with one another. In order to disentangle these method steps, the next three preferred embodiments of the method, as described below, may be used:
A first preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed at a temperature, at which the polymerization and/or crosslinking reaction is substantially hampered. The polymerization and/or crosslinking reaction rate is temperature-dependent. At a low temperature (at a high viscosity), the polymerization and/or crosslinking reaction is slower than the reorientation of the cholesteric helix, thus allowing the pitch of the cholesterically ordered material to be adjusted with limited or substantially no polymerization and/or crosslinking. At a high temperature (at a low viscosity), the polymerization and/or crosslinking reaction is faster than the reorientation of the cholesteric helix, thus allowing a polymerization and/or crosslinking with a limited or substantially no change in the pitch of the cholesterically ordered material. In conclusion, applying two irradiation steps, each at a different temperature, disentangles method steps b and c.
A second preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed by means of radiation which is substantially inert with respect to the initiation of the polymerization and/or crosslinking reaction. This method uses two irradiation steps, each step using radiation with a different wavelength. According to this embodiment of the method according to the invention, the layer is irradiated in step b with a desired pattern using radiation which is substantially inert with respect to the initiation of the polymerization and/or crosslinking reaction, thus allowing the pitch of the cholesterically ordered material to be adjusted with limited or substantially no polymerization and/or crosslinking. Subsequently, the polymerization and/or crosslinking is effected by means of actinic radiation having a suitable wavelength to initiate polymerization and/or crosslinking. Preferably, but not necessarily, said actinic radiation has a wavelength with a limited or substantially no influence on the pitch of the cholesterically ordered material. If said actinic radiation induces an additional change in the pitch of the cholesterically ordered material, then this must be taken into account when setting the pitch of the cholesterically ordered material in process step b.
A third preferred embodiment of the method according to the invention is characterized in that the irradiation in accordance with step b is performed in an atmosphere which substantially hampers the polymerization and/or crosslinking reaction. This method uses two irradiation steps, each step being performed in a different atmosphere. According to this embodiment of the method according to the invention, the layer is irradiated in step b with a desired pattern in an atmosphere comprising molecules, e.g. oxygen or nitrogen-monoxide, that quench the activated photo-initiators. Said quenching essentially deactivates the activated photo-initiators and thereby hampers the polymerization and/or crosslinking reaction. Consequently, the pitch of the cholesterically ordered material can be adjusted with limited or substantially no polymerization and/or crosslinking. Subsequently, the polymerization and/or crosslinking in accordance with step c is initiated by irradiation in a non-quenching atmosphere, e.g. nitrogen. Preferably, but not necessarily, the second wavelength used for the irradiation in accordance with step c, has a limited or substantially no influence on the pitch of the cholesterically ordered material. If said actinic radiation induces an additional change in the pitch of the cholesterically ordered material, then this must be taken into account when setting the pitch of the cholesterically ordered material in process step b.
The method according to the invention may yield a patterned layer of a polymer material having very interesting optical properties due to the selectively adjusted cholesteric ordering. Said method is particularly suitable for the manufacture of information carriers which may comprise (high resolution) data, text, images, emblems, logos, holograms or gratings.
The invention relates to a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, which layer comprises a quantity of a convertible compound which in its non-converted and in its converted state determines the pitch of the cholesterically ordered material to a different extent, and which layer comprises at least one region in which the pitch of the molecular helix differs from that of another region. This layer may be manufactured by means of one of the methods as described hereinbefore.
The invention also relates to an information carrier provided with a patterned layer of a polymer material having a cholesteric order, manufactured by means of one of the methods as described hereinbefore.
The invention further relates to a polarization conversion system provided with a patterned layer of a polymer material having a cholesteric order, manufactured by means of one of the methods as described hereinbefore.
The invention further relates to a color filter with a patterned layer of a polymer material having a cholesteric order, manufactured by means of one of the methods as described hereinbefore.
The invention further relates to a layer of a cholesterically ordered polymer material, in which the material is oriented in such a way that the axis of the molecular helix of the cholesterically ordered material extends transversely to the layer, and the pitch of the molecular helix varies transversely to the layer, said layer being manufactured by means of one of the methods as described hereinbefore.
The invention further relates to a polarizer comprising a layer having a cholesteric order as described in the previous paragraph, which layer is manufactured by means of one of the methods as described hereinbefore.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.